Thanks for the tip. With my 1983 300SD I got into the habit of plugging a Bescor voltmeter into the cigarette lighter plug. It reads 14-15 volts when the engine is running, reassuring me that the alternator is putting out some voltage and my battery won't be dead the next time I try to start the car.
Shortly after buying the car recently I had to clean off the battery terminals and ignition block connectors. The car had no power anywhere but the battery tested fine, excellent, in fact, because I had been charging it for the better part of a week. It tested 11 volts DC with a 200 amp draw at the local National Auto parts store. At least they were honest in not letting me buy a new battery.